1. Field of the Invention
This invention relates to an image processing device, an image output apparatus, and a storage medium storing an image processing program, and particularly to an image processing device, an image output apparatus, and a storage medium storing an image processing program for converting multi-gradation image data to low-gradation image data.
2 Description of the Related Art
An ink jet printer that has a recording head having plural nozzles arrayed therein and ejects liquid ink drops from the nozzles to record an image, has been broadly used. The ink jet printer has a simple structure, generates small printing sound, and can record a multi-gradation image such as a photographic image with high image quality.
Recently, a printer that has a relatively elongated recording head to enable high-speed print has been developed. Such a printer has an advantage that it can perform high-speed print, but it has a problem that since it is difficult to maintain uniformity of ejection characteristics (ejected ink drop volume and direction of ejection) of the nozzles because of the elongation, the image quality is lowered. Specifically, in a portion where large dots exist or where adjacent dots are close to each other, the density is higher than in other parts and black streaks occur. On the other hand, in a portion where small dots exist or where adjacent dots are away from each other, the density is lower than in other parts and white streaks occur. The unevenness of the ejection characteristics of the nozzles causes not only the black streaks and white streaks but also imbalance of density (density gradient).
In a PWA (Partial Width Array) ink jet printer that performs printing by moving a recording head into a main scanning direction while moving a recording sheet into a sub scanning direction, a multipass recording system is broadly employed. In the multipass recording system, a recording medium is minutely moved in the nozzle arraying direction of the recording head and the recording head is caused to scan plural times (i.e., perform multipass scanning) in a direction intersecting the nozzle arraying direction, thereby complementarily recording a thinned image in the same region on the recording medium with different nozzle groups to complete an image. This disperses the unevenness of the ejection characteristics of each nozzle of the recording head and the lowering of the image quality is prevented. However, in a so-called FWA (Full Width Array) ink jet printer that has an elongated recording head having a width substantially equal to the width of the recording sheet and performs recording by carrying only the recording medium while fixing the recording head, since multipass print cannot be carried out, the unevenness of the ejection characteristics of the nozzles is a particularly serious problem.
To address such a problem, as an apparatus that makes density unevenness and streaks inconspicuous in the case of using a printer having different output accuracies in the main scanning direction and in the sub scanning direction, an image processing device has been proposed that sets a reference threshold array in dither processing to provide partially acyclic threshold array characteristics in a region where the gradation shifts relatively from intermediate gradation to high gradation within a prescribed threshold range and to provide anisotropic threshold array characteristics that allow preferential connection and sequential growth of dots in a scanning direction where the output accuracy of the printer is relatively low (see, for example, Patent Reference 1).
Patent Reference 1: JP-A-2002-16803
However, if printing is performed on the basis of image data processed by using the above-described conventional technique of the image processing device, there is a problem that a linear pattern that does not exist in the original image, that is, so-called worm texture, tends to emerge in an intermediate gradation part of an image having scarce subtle changes of color. This is because the reference threshold array in dither processing is set to provide anisotropic threshold array characteristics that allow connection and sequential growth of dots. Moreover, in the above-described image processing device, the problem of density gradient due to the unevenness of the ink ejection characteristics is not taken into account.